More techincal information on how hovercraft work

 

The Body

Hovercraft are similar to aircraft in that they must generate lift and thrust at the same time (aircraft lift is generated by pushing or pulling the wings through the air, hovercraft lift is generated directly by a fan). As with aircraft, the weight of a hovercraft is critical - too heavy and it will need a large amount of power to produce the cushion and the thrust. The result is a large heavy engine and a bigger fuel tank meaning yet more weight! The hull construction and the engines used are chosen to minimize weight as much as possible. Aircraft style construction techniques, although ideal, are too expensive to be used to build recreational hovercraft. Most small hovercraft hulls are built from GRP (glass reinforced plastic - as used for small boats), plywood panels or foam composite. Each material has unique advantages and disadvantages. GRP is a very strong material and well suited to mass production but, to be effective, has to be relatively thick resulting in high weight, Plywood is cheap and quite strong but it is heavy and has a relatively short life (it's prone to rot or de-lamination from surface damage). Foam composites (foam panels sandwiched between thin GRP layers) is probably the best material currently available to home builders - it's slightly more expensive than GRP or plywood but gives good strength, is very lightweight and provides in built buoyancy. For mass production an injection molded composite foam/plastic would undoubtedly be the best and cheapest material for a hovercraft hull.

The Cushion

The way in which a hovercraft cushion actually works may not be immediately obvious from looking at it.

Basically, a fan pushes air under the craft - this air is prevented from escaping by the skirt. This results in increasing pressure under the craft. Once the pressure rises enough to become equal to the weight of the craft the hovercraft hull then rises up. The craft continues to rise up until the skirt is fully inflated and starts to lose it's seal against the surface - a gap then opens up all around the craft between the lowest point of the skirt and the surface. Air leaks out from this gap which in turn makes the craft settle down slightly. If more air is then pumped into the cushion then this skirt-to-surface gap opens up a bit more releasing the extra air. This small gap (about 10-30mm) acts like a pressure relief valve - no matter how much air is pumped into the cushion the craft won't rise much further. This simple self-correcting mechanism is what gives a hovercraft cushion it's inherent stability The actual pressure under the craft is always the same - it's the weight of the craft divided by the cushion footprint area.

The pressure under a typical hovercraft is only around 10 pounds per square foot (0.07psi or 4g/cm2). This is extremely low when compared to other vehicles.

The Skirt

There are two basic types of hovercraft skirt in common use on recreational hovercraft - the bag/loop skirt and the finger/segment skirt.

Bag skirts are probably better described as "tube" skirts as they look a bit like a round tube fitted around the outside edge of the hovercraft. Most of the fan lift air is pumped directly under the craft with a small amount diverted to inflate the bag skirt. A variant of the bag skirt is the full-flow bag where all of the lift air is fed through the bag and passes to the main cushion via holes on the inside bag surface. The bag is usually at the same or slightly higher pressure than the main cushion (still very soft). A bag skirted craft doesn't 'seal' onto rough surfaces as well as a finger skirt does but it's better at keeping the craft from tilting.

Finger skirts are a collection of "U" shaped (when viewed from the top), or three sided open bags with the base of the "U" attached to the outside edge of the hull and the top ends of the "U" attached to the inner part of the hull, The lift air is pumped into a distribution duct that runs around the outside edge of the craft - this duct has a hole on the bottom surface directly above the top of each "U" segment. Air is pumped out of these holes and forces the skirt segment outwards/downwards and also pushes it sideways against it's neighbors (looks like "UUUUUU"). Finger skirt craft are easily recognized by the "corrugated" look to the outside surface of skirt. The bottom edges of each "U" segment is curved inwards towards the centre of the craft and brushes against the ground to provide the cushion seal. The segments are made from a much lighter material than a bag skirt and, because they are individually deformable, they provide a better seal on rougher surfaces. The main disadvantage of being softer is that the hovercraft is easier to tip sideways.

Which skirt is better?

The simplest answer is neither! Both types have advantages and disadvantages

Finger skirt:

Easily damaged - especially on rough surfaces - skirt attachments are designed to break to protect skirt material.

Difficult to repair minor damage - material can't be glued

Easy to repair major damage (by replacing individual segments).

Expensive/complicated to make (large area of expensive material needed - material needs to be sewn into 3D patterns)

Softer ride on rough surfaces - less stable.

Leakier than a bag skirt - more power needed for the lift system.

Prone to 'plough-in'

Better suited to land/ river use where more compliant ride and where minor damage as less critical.

 

Bag skirt:

Tolerant of minor damage during use - you need to rip a big hole to lose cushion.

Difficult to repair major damage.

Easy to repair minor damage (by gluing/tying/patching).

Easy to make - low quantity of low cost material needed - very few joints (typically 3 to 6 per craft) - joints are glued.

Rougher ride - more stable

More efficient than segment skirt - better surface seal

Skirt can bounce on very smooth surfaces (skirt 'shudders').

Skirt can fill with water if craft left stopped on water (can be reduced by fitting flotation and drain holes).

Side skirt can 'tuck-under' when used on 'sticky' surfaces at low lift (the skirt catches on the surface and is pulled under the craft hull resulting in the craft tipping to one side)

Better suited to over water use where impact or surface damage is less likely and stability and safety are important.

A combination of these two skirt types can be seen in larger commercial hovercraft. It consists of a bag skirt with small segments attached to the bottom surface. This combines the advantage of the better conforming finger skirt with the stability and safety of a bag skirt.

The Thrust System

Hovercraft thrust is generated by either a propeller or a ducted fan. Both devices move air from in front of the craft and accelerate it out of the back. This accelerated mass of air then generates thrust which pushes the craft forwards. It works in exactly the same way that a light aircraft does. Propellers are a good choice for a cruising hovercraft as they are, in general, quieter and more efficient at the relatively low forward speed of a cruising craft (less than 60mph). Fans (of the type generally available) have to be fitted in a special duct and operated at high speeds to be effective (and are therefore noisy).

The Steering System

Hovercraft have the steering rudders or vanes placed directly in the thrust air flow at the back of the craft. For rudders to work effectively they must have a lot of air flowing over them. An aircraft has high airflow over it's control surfaces caused by it's relatively high forward speed - a hovercraft rudder has to work at zero forward speed. The hovercraft thrust is effectively re-directed by the rudders to provide steering.

Additional information